In U.S. Pat. Nos. 4,632,101; 4,721,103; 4,898,156; 4,968,315; 4,997,433; 5,037,422; 5,147,362; 5,207,679; and 5,217,486, there are disclosed a variety of anchors for attaching suture, bone and/or soft tissue to bone. The foregoing patents further disclose a number of installation tools for deploying the anchors disclosed therein. Complete details of the construction and operation of these anchors and their associated installation tools are provided in the above-identified patents, which patents are hereby incorporated herein by reference.
It will, therefore, be understood that anchoring devices such as those taught in the above-referenced patents generally comprise an anchor body, attachment means for attaching the desired object to the anchor body, and one or more barbs, pins, ridges, threads, or other bone-engaging means for holding the anchor body securely to the bone. Typically, the bone-engaging means are either manufactured separately from the body and then assembled to it, or they are machined/milled from the body itself.
Prior art bone-engaging means often take the form of several straight or curved cantilevered barbs, where the barbs may be elastically deformed to permit installation. This has been particularly true for anchors used in medical applications, where the need for high holding strengths has lead to the development of anchors having multiple cantilevered barbs. In any case, the body, the attachment means, and the bone-engaging means mechanically cooperate with one another to fasten a suture, bone portion, soft tissue, prosthesis, post or other object in a bone hole or bone tunnel.
Those skilled in the art will appreciate that known bone anchors generally experience a wide range of stresses during insertion into a bone hole or bone tunnel. Although some prior art anchors have attempted to incorporate polymeric materials in their construction, these anchors have, in practice, generally provided inadequate holding strength. As a consequence, known anchor bodies and known bone-engaging means have generally been made from high strength, biocompatible metals and metal alloys.
As the use of prior art anchoring devices has become more widespread, however, it has been found that known bone anchors suffer from a number of limitations. For example, forming both the anchor body and the bone-engaging means out of biocompatible metals and metal alloys increases the cost of manufacturing. Also, in many medical applications, it may be desirable to minimize the metal remaining in the patient's body after surgery.
Until now, however, the mechanical requirements placed on known anchors have generally limited the extent to which it was possible to incorporate alternate materials into the anchors.
For example, a large number of different biocompatible polymeric and bioabsorbable materials are currently available which are: (i) relatively low in cost; (ii) fully compatible with conventional fabrication methods; and (iii) capable of being absorbed into the body of the patient after surgery. Unfortunately, such polymeric and bioabsorbable materials generally do not possess the requisite mechanical properties that have been found to be necessary for adequate retention of the anchor in the bone hole or bone tunnel. In particular, it has been found that with "barb-type" prior art anchor designs, the anchor bodies cannot be formed out of these alternative materials since the materials have insufficient structural strength to hold the inboard ends of the prior art barbs fast. At the same time, the anchor barbs themselves cannot be formed out of these alternative materials since the materials have insufficient elasticity to permit anchor deployment and insufficient strength to hold the anchor fast to the bone.
With regard to the anchor bodies, it should be appreciated that substantial stresses may be induced in the prior art anchor bodies during both construction and use. For example, during the construction of some anchors, the inboard end of a curved barb is forced into a straight channel in the anchor body. This tends to induce significant stresses in the anchor body. Furthermore, in some anchors, significant stresses are induced in the anchor bodies during use when the barbs are deformed through engagement with a bone. Unfortunately, the aforementioned polymeric and bioabsorbable materials lack the structural strength of the aforementioned biocompatible metals, and hence anchors formed out of such polymeric and bioabsorbable materials using prior art designs tend to lack the requisite holding power.
It has been, therefore, impractical to incorporate the aforementioned polymeric and bioabsorbable materials into known anchor body designs.